When a combustible substance is burned, the quality of burning may be measured by sampling a combustion gas in the combustion chamber, the combustion gas being a heterogeneous mixture of a continuous gas phase and solid particles, and by measuring the amount of one or more substances in the sample, for instance, as a concentration. The gas phase generally comprises various gases produced in the combustion and gases entered into the combustion chamber from the environment.
The sampler may comprise, for instance, a probe locating in the combustion chamber and a pipe between the probe and a measuring part. A sample may be sucked by vacuum into the probe in the combustion chamber, wherefrom the pipe conveys the sample into the measuring part. Because the gas phase, especially, indicates the quality of combustion, the probe includes a filter that prevents access of at least major particles into the measurement. In addition, the filter prevents particles from blocking the pipe and the measuring part.
Sampling involves problems, however. Particles adhere to the filter in the probe all the time, whereby filtering capacity of the filter changes. This may change the measurement result constantly, and consequently it is difficult to obtain reliable information on the action of the combustion process. In addition, the filter may be blocked quickly, and it does not allow a gas sample to pass through at all, or only to an excessively small extent, and therefore the filter has to be replaced frequently. Replacing may be cumbersome, however, and it may require that the combustion process be extinguished. Even though the combustion process would not have to be extinguished, the filter replacement interrupts sampling and measuring, and thus prevents continuous sampling and measuring of the combustion process.
The replacement interval of the filters may be prolonged by cleaning the filters. Even though it may not be necessary to displace the filter for cleaning it, but the filter may be cleaned, for instance, by pressurized gas against the sampling, yet the cleaning operation interrupts the sampling and the measurement frequently.
On account of the above, there is a need for improved sampling of a heterogeneous mixture containing a gas phase and particles of a solid.